This invention relates to heat-treatment of shaping metal member used in an apparatus for plastic metal-working such as rolling, drawing, extrusion, seamless tubing, contour forging and die casting, and more particularly to a process of oxidative heat-treating the rubbing surface of the shaping metal member which is abrasively contacted with a semifinished metal product which slides over said surface with high pressure. By this oxidative heat-treatment, a heat-resistant metal oxide layer is deposited on the surface of shaping metal member, reducing the depletion thereof caused by the sliding of the semifinished metal product.
Brief description is now given of one form of the above-mentioned working of plastic metal with regard to seamless tubing for example.
The process for making seamless tube comprises heating a raw rod steel material to a temperature of about 1,200.degree. C. for softening; boring the softened steel rod with a piercing plug; broadening the inner diameter of the bore with an elongating plug; causing the workpiece to have an inner diameter and thickness approximately those realized in a final size by applying a rolling plug; and finishing at last a seamless tube with a reeler plug.
Seamless tubing mill's tools such as the above-mentioned various plugs are subjected to high pressure and temperature in practical operation. Therefore, abrasion, melting loss and seizing take place on the semiwork sliding surface, resulting in deformation or damage which occurs quickly in the tubing mill tools. Since such a deformed or damaged tubing mill tool is no longer useful for practical purpose, the tubing mill tube is a highly consumable article which has to be replaced often by a new one.
The seamless tubing mill tools are generally made of wear resistant low alloyed cast steel or highly heat resistant high carbon stainless cast steel. Yet, the tubing mill tools formed from the low alloyed steels are capable of being applied only a few times. Though applicable more often than such mill tools, the mill tools formed from the stainless cast steel are too expensive for practical application.
A known process (Japanese patent application publication No. 3884/1975) for extending the effective life of a piercing plug made of alloyed cast steel comprises heat-treating a piercing plug in an atmosphere of an ordinary fuel burning gas at a temperature of about 1,000.degree. C. for several hours to deposit a thin metal oxide layer on the surface of the piercing plug, and fixing the thin layer to the surface by slowly cooling the piercing plug, thereby improving the heat and wear resistances of the piercing plug for the extension of its life.
However, the thin metal oxide layer thus obtained is ready to fall off the surface of a metal substrate or fails to be deposited on said surface in a thin layer of uniform quality. Therefore, the above-mentioned process can not be regarded as capable of prominently extending the life of a piercing plug.
FIG. 1 is an 80-times magnified microscopic photograph of a longitudinal section of a piercing plug made of low alloyed cast steel containing 0.3% C, 3% Cr and 1% Ni, heat-treated for 3 hours in an atmosphere of ordinary CO-free completely burned gas at a temperature of 950.degree. C., cooled to 500.degree. C. in the furnace used, and quenched by air cooling. FIG. 1 shows that the above-mentioned heat-treatment gave rise to the formation of double layers consisting of an inner metal oxide layer 11 and outer metal oxide layer 12 on the surface of a metal substrate, over which a semifinished product is expected to slide abrasively. The outer metal oxide layer 12 mainly consisting of Fe.sub.3 O.sub.4 and containing a small amount of Fe.sub.2 O.sub.3 is brittle and readily damaged by abrasion, and has a greater thickness than the inner metal oxide layer 11.
The inner metal oxide layer 11, mainly consisting of FeO.Cr.sub.2 O.sub.3 and containing a small amount of FeO, is tougher than the outer layer 12, but is ready to be damaged by abrasion due to its small thickness. If it is tried to increase the thickness of the inner layer 11 by adjusting the conditions of heat-treatment, then the outer layer 12 increases in thickness to a corresponding extent, presenting difficulties in ensuring the dimensional precision of the piercing plug.
FIG. 2 is an 80-times magnified microscopic photograph of a longitudinal section of a rolling plug which was made of high carbon cast stainless steel containing 1.2% C, 17% Cr and 2% of Mo and W together, heat-treated in an ordinary CO-free completely burned gas at a temperature of 1,070.degree. C. for 4 hours, cooled in the furnace used to 880.degree. C., kept at this temperature for 1 hour, and quenched by air cooling.
FIG. 2 shows that the above-mentioned heat-treatment produces a single layer 21 of metal oxide on a metal substrate 20. In the case of high carbon cast stainless steel, the double layers of metal oxide which is shown in FIG. 1 are not produced on the surface of heat-treated plug. The metal oxide layer 21 of FIG. 2 has the same composition as the inner metal oxide layer 11 of FIG. 1 which is heat-insulating and wear resistant. The metal oxide layer 21 has an appreciably great thickness on the average, but is regrettably of non-uniform quality, and is sparsely deposited on the metal substrate 20. Therefore, the metal oxide layer 21 also fails to sustain a long period of resistance to abrasion.
For reference, the upper dark portion 13 of FIGS. 1 to 8 denotes a resin used to fix a sample in place to obtain a cut plane surface of the sample. The black portion 22 of FIGS. 2 to 8 which appears below the dark portion 13 or in the intermediate part thereof represents a gap produced in the resin when it shrinks due to hardening.